A comparator is a device used to look for parallax motion, proper motion, asteroids, or variable
stars by quickly alternating between views of two photographic plates taken
at different times; also known as a blink comparator or blink microscope. Changes in the configuration of the objects
on the plates, such as the movement of a suspected planet, are detectable
by their relative motion, or new objects become apparent by their alternate
appearance and disappearance.
|Clyde Tombaugh at work on the blink comparator at
the Lowell Observatory, c. 1938. Credit: Lowell Observatory
A similar instrument is the stereocomparator, which highlights
objects that have changed position by their stereoscopic appearance.
EQUIPMENT AND TECHNIQUES